In Southern Alberta, Canada, heavy oil is sometimes recovered from unconsolidated sandstone formations using a technique called cold production. The operator of the well aggressively perforates the well and purposefully produces formation sand along with the heavy oil. This technique pulls sand from the formation, increasing oil mobility and formation permeability for improving the flow of viscous oil to the well. Typically sand production is high upon well completion and for a period thereafter. Often a sump is used, located below the perforations for collecting the first inrush of sand. Conventional pumps such as progressive cavity pumps (PCP) or reciprocating rod pumps can be used with sand concentration less that about 20%. PCP's are more tolerant of sand than are reciprocating pumps. However, excessive sand concentrations still persist in some wells. The sump and well can sand-in and sand slugs can pump umps and halt production until an expensive and time-consuming workover clears the sand. Usually, by that time PCP failure has occurred. If a low cost reciprocating pump jack or rotary top drive is used to operate the pump, an expensive service rig must be called in to pull the pump or flush the PCP. Even more costly is to maintain a service rig at the well.
For removing excessive sand and for emptying a sump, prior art techniques include using a reciprocating barrel pump with a lower, sand-collecting tailpiece. This process is termed "bailing". The pump is located above the tailpiece. The pump draws solids and liquid into the tailpiece. Solids settle and liquid continues upwardly to spill back into the annular space between the pump barrel and the wellbore. Solids collect until the tailpiece is full and it is pulled out of the well.
In U.S. Pat. No. 4,711,299 to Caldwell, a reciprocating barrel pump is applied to a well with solids, and more specifically a well having undesirable liquids which need to be pumped out of the well. The pump barrel is suspended from a tubing string. An upper check valve is fitted at the top of the barrel. A stationary piston having a hollow piston rod hangs from and below the barrel. A tailpiece is once again provided which hangs from the piston rod. A lower check valve is fitted at the bottom of the piston rod, adjacent or within the tailpiece. When the barrel reciprocates, sand and liquid is drawn into the tailpiece. The entrance to the piston rod is purposefully narrow to cause high velocity liquid flow. Solids are not intended to pass above the lower check valve. In some implementations a screen rejects solids. Liquid continues up through the piston rod and out of the well as required.
Bailers do not pump sand to the surface and must be pulled from the well to remove sand and return the conventional pump to the well.
Others, such as Site Oil Tools and Arrow Oil Tools have converted conventional bailers to systems which pump sand and liquid to the surface by the addition of an anchor. Conversion from liquid only bailer to pumps handling sand as well introduces several operational difficulties. The travelling valve is located at the top of the piston rod which means they can be in the order of 12 feet from the standing valve. Suction created by these arrangements is poor, resulting in loss of pumping. The small bore through the piston rod causes high pressures in the barrel when the piston and piston rod stroke downwardly. At these pressures, sand separates from the oil and pack up in the barrel, and also form wads or balls of sand which can bridge the production tubing or block elbows and valves at the surface. Further, the sand causes significant wear on the moving components of the pump.
Typically, bailers and bailer conversions use "V"-cup packing, such as that use in wellhead rod seals). The packing-type seals are virtually incapable of sustained use when exposed to sand.
Production pumps, which utilize reciprocating rods, seriously impede the flow path to the surface particularly when the rods alternately move contrary to the desired flow of sand-laden oil, cause fall out of sand, and suffer delayed rod fall. Further, the rod pumps and known reciprocating pumps generally use pistons having elastomeric seals snugly supported in individual piston grooves, subject to being rendered ineffective with sand. As shown in a prior art pump in FIG. 1, the piston can be 2-4 feet long, the travelling valve and standing valves are widely spaced and no means are provided for excluding sand.
Sands from the above-described wells are very fine and tend to pack up in the individual piston grooves and render the seals ineffectual. The sand may be likened to a lapping compound, causing high wear and ultimately resulting in barrel failure.
The problems of sanding in heavy oil wells is discussed in a 1995 paper presented at a Heavy Oil Symposium in Calgary, Alberta, "Practical Requirements for Sand Production Implementation in Heavy Oil Applications", by Dusseault, M. B. et al., publication SPE 30259. The authors identify quick removal of bailers and the resulting suction as one of the causes of re-sanding. The authors further suggest improvements such as washing techniques, jet pump to surface techniques, and slow withdrawal of bailers with fluid replacement.
In this paper, the aforementioned authors acknowledge the superiority of PCP over reciprocation pumps, yet describe PCP failures and reiterate the need for effective sand removal and sand-tolerant pumps.
There is thus an expressed need for a pump which replaces the known bailer or bailer conversions, rod pumps and progressive cavity pumps for pumping liquids to the surface from wells having liquids associated with fine solids, particularly cold production heavy oil wells.